1. Field of the Invention
This invention relates generally to the testing of elements, such as receivers, used in communications links, for example fiber optic communications links. More specifically, in cases where an element, such as receiver, includes a linear front-end and a separate back-end (host) that includes equalization, this invention relates to separate testing of the front-end and/or back-end.
2. Description of the Related Art
Optical fiber is widely used as a communications medium in high speed digital networks, including local area networks (LANs), storage area networks (SANs), and wide area networks (WANs). There has been a trend in optical networking towards ever-increasing data rates. While 100 Mbps was once considered extremely fast for enterprise networking, attention has recently shifted to 10 Gbps, 100 times faster. As used in this application, 10 Gigabit (abbreviated as 10G or 10 Gbps or 10 Gbit/s) systems are understood to include optical fiber communication systems that have data rates or line rates (i.e., bit rates including overhead) of approximately 10 Gigabits per second. This includes, for example, LRM and SFF-8431, a specification currently under development by the SFF Committee that will document the SFP+ specifications for 10G Ethernet and other 10G systems. While 10G systems serve as convenient examples for the current invention, the current invention is not limited to 10G systems. Examples of other systems to which the current invention could be applied include Fibre Channel systems, which currently operate at speeds from 1 Gbps to 10 Gbps, as specified by the Technical Committee T11, a committee of the InterNational Committee for Information Technology Standards (INCITS).
Regardless of the specific data rate, application or architecture, communications links (including optical fiber communications links) invariably include a transmitter, a channel and a receiver. In an optical fiber communications link, the transmitter typically converts the digital data to be sent to an optical form suitable for transmission over the channel (i.e., the optical fiber). The optical signal is transported from the transmitter to the receiver over the channel, possibly suffering channel impairments along the way, and the receiver then recovers the digital data from the received optical signal.
Recent developments in 10G optical communications have included the use of Electronic Dispersion Compensation (EDC) in receivers to extend range. For example, the IEEE 802.3aq standards committee is developing a standard (10GBASE-LRM or simply LRM) for 10G Ethernet over multi-mode fiber over distances of up to 220 meters using EDC. This standard is in a draft state, currently documented in IEEE Draft P802.3aq/D4.0, Draft amendment to: IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements, Part 3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications, Amendment: Physical Layer and Management Parameters for 10 Gb/s Operation, Type 10GBASE-LRM, referred to herein as IEEE 802.3aq/D4.0 or LRM, and incorporated by reference. The use of EDC in the receiver allows communication over longer distances and/or use of lower cost components. Some of the added waveform distortions can be corrected in the EDC receiver. However, an EDC receiver generally requires linear signaling between the dispersion mechanism (e.g., the fiber) and the equalizer that performs the EDC function.
Standards play an important role in networking and communications. Since components in the network may come from different vendors, standards ensure that different components will interoperate with each other and that overall system performance metrics can be achieved even when components are sourced from different vendors. For example, standards for receivers can be used to ensure that, when compliant transmitters are combined with a compliant channel and a compliant receiver, the overall link will meet certain minimum performance levels. As a result, manufacturers of receivers typically will want to test their receivers for compliance with the relevant standards as part of their qualification or production processes.
In the context of approaches such as LRM, the receiver may be divided into a module that contains the O/E conversion (e.g., a photodetector) and a host for the module. For convenience, these will be referred to as the receiver module and host receiver, respectively. If the EDC capability resides in the host receiver, then the O/E receiver module preferably is linear. However, the O/E module and the host may come from different vendors. Alternately, a system integrator may purchase O/E modules from a supplier for integration into its own host. Regardless of how it is assembled, in order for the overall receiver to be compliant, the O/E receiver module and corresponding host receiver together must be compliant. Current standards such as LRM may define a single standard for the receiver as a whole, without allocating requirements between the receiver module and the host receiver. In addition, EDC drives the need for a linear O/E module but EDC is a relatively new proposal. Older standards may provide separate specifications for the O/E module and host, but they typically are oriented towards hard-limited O/E modules rather than linear ones. In hard-limited O/E modules, if the input signal is above a threshold (nominally set at the average value of the signal), the output is at a logic one (high) level; whenever the signal is below the threshold, the output it at a logic zero (low) level. Hard-limited O/E modules are not linear.
Thus, there is a need for a receiver testing and compliance measurement technique for communications links, including, for example, 10G optical fiber communications links, where the linear receiver module and the host receiver (which includes EDC) can be tested and qualified independently.